In recent years, the water absorbent resins have been being utilized extensively as a main ingredient for such sanitary materials (absorbent articles) as disposable diapers, sanitary napkins, and incontinence pads with the object of absorbing humors (urine and blood). As concrete examples of the water absorbent resins mentioned above, polycarboxylic acid water absorbent resins including such polyacrylic acid water absorbent resins as partially neutralized crosslinked polyacrylic acids and hydrolyzates of starch-acrylonitrile graft polymers may be cited. Among other water absorbent resins, the polyacrylic acid water absorbent resins are used in particularly large amounts because of low prices and excellent solid state properties.
Also in recent years, the water absorbent resins have come to find growing adoption for agriculture and horticulture besides sanitary materials by virtue of their low prices and their ability to retain water. The polyacrylic acid water absorbent resins mentioned above, for example, are being utilized as water retaining materials for tree planting, water saving cultivation, and sandy field cultivation by virtue of their ability to retain water (refer to the JP-A 1983-42602, the JP-A 1988-68026, and the JP-A 1989-51028, for example). Further, the water absorbent resins using ammonium salts of polyacrylic acids have been disclosed with a view to providing water retaining agents which exert no adverse effects on plant growth (refer to the JP-A 1987-273283, for example). The JP-A 2000-139208 has disclosed a technique which concerns polyacrylic acid water absorbent resins improved to overcome the obstruction to plant growth.
It has been demonstrated, however, that when the conventional polyacrylic acid water absorbent resins disclosed in the JP-A 1983-42602, the JP-A 1988-68026, and the JP-A 1989-51028, for example, are used as water retaining agents for plant growth, they exert adverse effects on the growth of plants and induce particularly serious hindrance to rhizogenesis and growth of roots (Kazuo Kawashima et al., “Effects of highly water absorbing polymer substances on initial growth of crops,” Sakyu Kenkyu, 31 (1), 1-8, 1984). It has been also demonstrated that calcium is indispensable to the rhizogenesis and the growth of roots in plants (Sunao Takakura, “Growth of plants and environment,” Nobunkyo, Table 5-2, page 162).
Thus, the use of the technique disclosed in the JP-1987-273283 which resides in simply changing the counterion of carboxylic acid from the conventionally generally used sodium salt to the ammonium salt has been incapable of repressing the conspicuous obstruction to rhizogenesis and growth of roots. When the conventional polyacrylic acid water absorbent resin is used for tree planting, therefore, the amount of the resin to be used is restricted to only not more than several wt. % based on the amount of soil, for example, lest the seedlings or seeds of a plant should suffer serious hindrance of growth due to direct contact with the polyacrylic acid water absorbent resin. Thus, the decrease in frequency of irrigation and the effect of water retention have not been satisfactorily manifested. Especially, it did not use for growing a plant in a state of swelled hydrogel without mixing other carrier. Therefore it did not use as a water retaining carrier of plant growth for interior such as a propagation by cutting, hydro culture, flower arrangement.
The technique disclosed in the specification of U.S. Pat. No. 6,286,254 prevents the inhibition of growth by preparing a water retaining carrier for plant growth which includes a hydrogel forming polymer having a calcium ion absorbing capacity of less than 50 mg based on the dry weight and exhibiting a absorption capacity of not less than 100 times the original volume in deionized water (at room temperature of 25° C.) and avoiding to deprive the plant of calcium necessary for the growth of the plant. It is, however, described in the specification of U.S. Pat. No. 6,286,254 that the hydrogel obtained by swelling the cross-linked copolymer of acrylamide and acrylic acid or the acrylic acid water absorbent resin with water suffers a conspicuous decline of the water absorbing speed because it is doped with such a water-soluble- or a water-easy soluble-polyvalent metal as calcium chloride. Since the hydrogel thus suffers the decline in the water absorbing speed in spite of a high water absorbing ratio, the irrigation water can not be kept in a soil by decreasing the amount of water absorbed in the absorbent resin. Therefore, the actual use thereof in soil induces a decrease in the efficiency of irrigation due to the loss by scattering.
The technique disclosed in the JP-A 2000-139208 prevents the inhibition of the growth of a plant by using an acrylic acid water absorbent resin having a calcium ion absorbing capacity in the range of 0-100 mg per 1 g of the dry weight and a chlorine ion content in the range of 0.07-7 mmols per 1 g of the dry weight as a water retaining carrier for a plant and consequently promoting the absorption of calcium in the plant. The technique disclosed in the JP-A 2000-139208, however, suffers a large decline of the ability to absorb water because of a cross-linking with a polyvalent metal similarly to the technique disclosed in the specification of U.S. Pat. No. 6,286,254 because the water absorbent resin in a highly hydrated state undergoes mixture with such a polyvalent metal as calcium chloride and consequently suffers the polyvalent metal to be distributed uniformly in the whole water absorbent resin. Particularly the water retaining material incurs a decline in the water absorbing speed. When this water retaining material during its actual use is mixed with soil and then irrigated, therefore, it entails the problem of losing the efficiency of irrigation because it fails to absorb sufficiently the water used for the irrigation and consequently suffers the water to flow out. The technique disclosed in the JP-A 2000-139208 exposes the devices and the equipment used for actual commercialization to heavy load and damage because the water retaining material contains a chlorine ion. Further, the addition of deliquescent calcium chloride, for example, results in impairing the hygroscopic fluidity (Anti-caking property/Anti-blocking property) and the fluidity of the water retaining material and accordingly posing the property of handling the powder as a problem. Because of the presence of the chlorine ion, the water retaining material causes problems at firing it, and others it is suffered to accumulate and consequently pose an environmental problem on being repeatedly scattered on the soil, for example. Thus, in the prior art, since the water absorbing characteristic manifested by the water retaining material for plant growth as the water retaining material and the growth promoting characteristic manifested thereby on the plant being grown contradict each other, it has been extremely difficult to provide a water retaining material for plant growth which reconciles these two characteristics.
The task which this invention aims to fulfill resides in affording a water retaining material for plant growth possessing the water absorbing characteristic and the plant growth promoting characteristic which have been contradicting each other hitherto and consequently providing a novel water retaining material for cultivating plant which is veritably excellent in the efficiency of irrigation and useful for soil conditioning and tree planting. Furthermore, it is to provide a novel water retaining material for plant growth which enable to use in a soil in a high concentration, thereby cultivating a plant in a swelled gel directly.